Light emitting diodes (LEDs) are based on a forward biased p-n junction and have recently reached sufficiently high brightness levels that they are now suitable for new solid-state lighting applications as well as replacements for projector light sources. Entry into these markets has also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. In particular, applications in solid-state lighting require that LEDs exceed efficiencies currently achievable by alternative fluorescent lighting technologies.
Back Light Units (BLU) for liquid crystal display (LCD) panels are key elements in the performance of an LCD panel. Currently, most LCD panels employ compact cathode fluorescent light (ccfl) sources However, these sources suffer from several problems, including poor colour gamut, environmental recycling and manufacture issues, thickness and profile, high voltage requirements, poor thermal management, weight and high power consumption In order to alleviate these problems LCD manufacturers are implementing LED BLU units. These offer benefits in many areas, including colour gamut, lower power consumption, thin profiles, low voltage requirements, good thermal management and low weight.
Current LED BLU systems distribute LEDs across the back surface of the LED as demonstrated in U.S. Pat. No. 7,052,152. For small displays these are generally low cost designs. However, for larger LCD panels, for example greater than 32 inches, the number of LEDs required to distribute the light evenly across the back of the LCD panel no longer makes this approach cost effective In U.S. Pat. No. 7,052,152 it is proposed that 124 LEDs are employed for a 32 inch LCD panel display.
For some applications, a more isotropic or Lambertian light distribution from an LED is desired and one technique used to achieve this is to roughen the surface through which the light merges. A certain degree of roughness is inherent depending on the LED fabrication process. However, by using techniques such as etching a controlled degree of increased surface roughness can be achieved to improve the scrambling and uniformity of light emerging from the LED.
US2006/0181899 and US2006/0181903 disclose an optical light guide or waveguide arranged to evenly distribute diffused light across the complete LCD panel surface. The light is coupled into the light guide by using side-mounted LED optical sources. Side mounting is beneficial as the number of LEDs required is dramatically reduced per unit area of backlit LCD, when compared with direct LED backlighting. It is proposed that a 32 inch LCD panel can be side lit with as few as 12 LED sources. However, to maximise the light diffused onto the LCD panel the most optimal coupling of the LED light into the light guide is required.
Another area for application of high-brightness LEDs is in light engines for front and rear projectors. Conventional High Intensity Discharge (HID) type projector light engines have always been hindered by low efficiency and short lifetime, resulting in slow adoption into consumer markets. In this particular application the Etendue value of the light source needs to be smaller or matched to the microdisplay Etendue value. This compatibility is very important for improving the overall system efficiency of the complete light projection engine. Additionally, high total luminous outputs and low power consumption are also very important, especially in applications where large rear projection screens (bigger than 50 inch) and front projection systems are required. Low power consumption is desirable in order to minimise thermal management issues Light from a set of red, green and blue single-colour LED sources with small Etendue values are multiplexed to produce the desired colour in the projection system. This eliminates the need for colour wheels and the associated additional costs. The Etendue value E is calculated according to the following equation;E=π×A×n2×sin(α)where E is the Etendue of the light source. A is the surface area of the light emission device, and α is the half-angle of the light source. Hence, it can be seen that for projection applications, the degree of collimation of the light emitting source is a key factor and that reducing the half-angle of the source dramatically improves the overall efficiency of the light engine.
The overall efficiency of an LED can be quantified by three main factors, namely internal quantum efficiency, injection efficiency, and extraction efficiency. One of the main limiting factors reducing the efficiency of light extraction from an LED is total internal reflection of the emitted photons and their trapping in the high refractive index of the epi-material forming the LED. These trapped waveguide modes propagate in the LED structure until they are scattered or reabsorbed. The thickness of the LED structure determines the number of modes that can be supported.
U.S. Pat. No. 5,779,924 and U.S. Pat. No. 5,955,749 both describe the use of photonic crystal patterns defined in the semiconductor layers of the LED to affect the way light propagates through the epi-structure. The photonic band structure formed allows the trapped modes to be extracted out and hence increase the extraction efficiency and ultimately the total external efficiency of the LED. The use of photonic crystal structures in an LED is advantageous over other light extraction techniques as they scale with the active surface area of the LED and hence provide an ideal means for improving the light extraction for a large-area high-brightness LED die. The scaling of the LED size is important for solid state lighting applications where absolute luminous output is required. However, the total light extraction for many of these photonic crystal LEDs is not as high as for more conventional surface-roughened LEDs.
U.S. Pat. No. 6,831,302 and US2005/0285132 describe processes for fabricating light-emitting diodes with photonic crystal structures using Gallium Nitride (GaN) based materials. In both cases the processing involves many complex and expensive steps that ultimately affect the yield and cost of the LED wafers. In particular, U.S. Pat. No. 6,831,302 describes a fabrication process that involves the following steps: growing an n-GaN layer on a lattice matched single crystal wafer, an active QW region, and a p-GaN layer, followed by eutectic bonding of a sub-mount or substrate on the top surface, wafer flipping, lift-off from the growth wafer (using a technique such as laser lift-off), surface polishing to provide an optically-smooth surface (using a process such as chemical mechanical polishing), definition of a photonic crystal pattern on the surface (by a process such as nano-imprinting, lithography, or holography), and finally photonic crystal pattern transfer into the GaN material using a suitable dry (e.g. RIE or ICP) or wet etch of the GaN.
One of the complex processing steps involved is the polishing of the wafers, which can adversely affect the yield due to the difficulty in controlling the surface quality across the complete wafer. Small scratches across the surface may affect the current spreading across the LED wafer and ultimately provide a path to short circuiting the complete LED or adversely affecting the forward voltage. Additionally, the thickness of the GaN epi-structure is important for the effective design of the photonic crystal extraction pattern. The high refractive index acts like a highly-multimode waveguide, whereby the thickness determines the number of modes residing in the LED heterostructure. By using a polishing process, poor control over the absolute thickness of the LED structure is achieved, which ultimately affects the overall output of the LED wafers from one processing batch to another. Another complex fabrication step is the definition of small-scale first-order photonic crystal features in the range of 300 nm to 500 nm for the pitch of the features and in the range of 200 nm to 400 nm for the diameter of the hole features. Such patterns are currently defined using either nano-imprinting or holography. The former technique is currently not a proven technique for such small-scale features on LED wafers and only low yields are achieved. In addition, the technique suffers from a higher cost for small manufacturing volumes. The latter lithography technique suffers from complex alignment and stability as well as low volumes.
There is therefore a need for a new type of surface patterned LED, which performs better than conventional surface-roughened or photonic crystal LED device and which can be fabricated in a simple and cost-effective manner.